Microwave Doppler radar systems are systems in which a stable microwave signal is generated by a microwave source and transmitted from an antenna into the field of a moving target. A reflection signal is generated from the moving target in the system's field of view. The reflected signal is received by the receiving antenna of the system, and is mixed against the signal being broadcast resulting in a difference frequency component, which is known in the art as the Doppler Frequency Shift. The difference frequency is proportional to the relative speed between the target and the radar system along the line of sight. In general, accuracy of the speed measurement depends on the short term drift of the microwave source frequency over the duration of the time of flight between the transmitted signal and later mixing of the received reflected signal. Accuracy of the measurement also depends on the ability of the processing circuitry to convert the received Doppler signal into a representative number.
Numerous radar guns are commercially available which calculate speed using the Doppler shift. Typically, these guns use a Gunn diode in a resonant housing feeding a horn antenna. The Gunn diode serves as the microwave source as well as the mixer.
Several Doppler type motion detector units have been invented which use printed circuit board technology, but all have distinct disadvantages compared to the fabrication techniques used in the present invention. Other advantages of the various embodiments of the present invention are apparent in the following discussion.
In some implementations, the antenna is outside of its housing and is connected to the associated microwave circuitry with wires. From an assembly point of view, this is a distinct disadvantage to the integrated approach used in the present invention.
Some motion detection units sense motion using a Doppler Shift Frequently, the construction of these units uses a three-layer board wherein energy is transferred from one side of the board to the other by coupling through resonant slots in the ground plane. A three-layer board is generally formed by gluing together a two-layer board to a single layer board. The use of resonant slots avoids the use of via holes, but makes control of the height from the internal ground plane to one of the outside circuitry planes difficult. The thickness of the glue has to be controlled very precisely, since microwave circuits are extremely dependent on the height of the ground plane to the top conductor.
Some low cost Doppler motion sensors are constructed on two-layer FR4 type circuit boards with two 1.times.2-patch antenna arrays and a ground plane on one side of the board, and the microwave circuitry and a ground plane on the other side. Putting the circuitry adjacent to the antennas has the disadvantages of increasing board size, and decreasing the RF sensitivity since longer lengths of lossy microwave transmission lines are needed to go from the circuitry to the antenna.
For the foregoing reasons, there is a need for a Doppler Radar motion measuring unit which can be constructed with low cost, has attractive DC to RF efficiency for longer battery lifetime, has minimal size, allows the ability to add RF gain stages to increase maximum range, and which uses two antennas to allow a larger amount of gain to be added than if a single antenna was used.